Internal combustion engines comprise pistons which reciprocate within cylinders. Up and down movement of the pistons is converted to a circular output motion through a crankshaft. The pistons are connected to the crankshaft with connecting rods. The crankshaft and connecting rods are housed in a crankcase. In operation, compressed combustion gases can leak past the pistons and enter the crankcase. These gases are normally referred to as "blow-by" gases. The crankcase is enclosed and thus accumulating blowby gases and the expansion of gases through heating can cause pressure to build up in the crankcase. Further, the gas-space in the crankcase is contiguous with the bore of the cylinders which is below the pistons. Up and down movement of the pistons within the cylinders varies the volume between pistons and the crankcase which can affect the pressure in the crankcase.
Conventional multi-cylinder engines use alternating piston movement, one piston rising while another descends, thus balancing the displaced volume and minimizing the variation in crankcase pressure.
Harley Davidson ("Harley") motorcycle engines have two cylinders and differ from conventional multi-cylinder internal combustion engines in that the two connecting rods run on a single crank. In other words, both of the pistons stroke upwardly together and stroke downwardly together synchronously. The piston movement is counterbalanced with two large crankshaft-mounted flywheels. Accordingly, on the downstroke, both pistons simultaneously move downward and diminish the volume available for the contained gases. The pistons compress the gases in the crankcase, pressurizing the crankcase gases to above atmospheric pressure ("positive" pressure). On the upstroke, both pistons simultaneously decompress crankcase gases, reducing the crankcase pressure to below atmospheric pressure ("negative" pressure). Accordingly, Harley engines produce large alternating positive and negative crankcase pressures.
Most visibly, high crankcase pressure is associated with leakage from crankcase seals.
On Harley engines produced earlier than 1993, the management of crankcase pressure is performed with a breather gear, driven and timed by the crankshaft. The gear is typically set to open and vent crankcase gases between 10.degree. before top dead center (before TDC) through 75.degree. after bottom dead center (after BDC). The breather gear vents crankcase gas to a separate camshaft chamber where the bulk of the oil mist is knocked out. The de-misted gas is then directed through a breather hose to the air cleaner. Due to the inherent physical limitations, the gear timing venting is not optimal at all engine speeds and throttle conditions. High crankcase pressures still result.
In another aspect, the oil system of Harley engines is also rather unique, being of the dry sump variety and having a separate oil tank. A scavenging pump collects oil from the bottom of the crankcase, routes it through an oil filter and on to an oil reservoir or tank. Oil flows under gravity feed from the oil tank to a feed pump which delivers oil to the engine components. Baffles in the camshaft chamber separate returning oil mist from crankcase gases before the oil collects at the scavenging pump. There are two aspects of this system which are sensitive to crankcase pressure. Firstly, excessive suction in the oil tank, the head space of which is in communication with the crankcase, adversely affects the supply of oil to the feed pump. Secondly, lack of a head of oil at the inlet of the scavenging pump and excessive suction in the crankcase can starve the scavenging pump of oil. In short, the excessive suction can result in oil-related engine failure.
In post-1993 Harley engines the breather hose has been relocated, from the crankcase, to each of the two rocker housings. Crankcase gases and pressure communicate with the rocker housings through the push rod tubes. A one-way check valve mounted within each rocker housing releases excessive crankcase pressure into the housing. The check valve is an "umbrella-type" valve having a port or ports blocked with an elastomeric umbrella valve head. The umbrella is normally closed over the port to prevent inflow of gases into the crankcase. Pressure flexes the umbrella off of the port so as to release gases from the engine. A small bleed hole is provided which permits collected oil to drain back to the crankcase. It is apparent that the bleed hole can also permit some gases to return to the crankcase. In the stock arrangement, a port directs the gases directly into the air cleaner. As an accessory, after-market cross-over tubing can be installed between the two rocker housings. A "tee" in the tubing directs the crankcase gases to a discharge tube and filter which removes oil mist.
Others have utilized crankcase breather valves in the context of conventional 4-stroke engines. The valves are known for reducing oil seal leakage by releasing excess pressure and forming a predominately negative pressure in the crankcase. Several breather valves use the "umbrella-type" valve heads ("umbrella"). For instance, in U.S. Pat. No. 5,067,449 to Bonde and U.S. Pat. No. 5,205,243 to Buchholz, disclose crankcase breather assemblies. An assembly is inserted into a port formed in the crankcase. The assembly incorporates an outer groove which retainably engages a lip formed in the port. The assembly further incorporates an umbrella which covers and seats over a circular array of ports. The umbrella is normally-closed so as to ensure only one-way flow through the ports. In U.S. Pat. No. 5,027,784, Osawa et al. improved the operability of an umbrella-type valve by interposing a washer between the umbrella and the ports. The washer reduces over-flexing and premature failure of the umbrella. Despite the presence of the washer, Osawa's umbrella still rests in the normally-closed position.
Thus check valves of the umbrella-type are known and they are all of the normally-closed, one-way variety. Accordingly, while these valves permit flow out of the crankcase on over-pressure, they do not permit any gas flow back into the crankcase, except for a small amount of sealing hysterisis.
While the synchronous piston movement in a Harley Davidson engine can benefit from a reduction of maximum crankcase pressure, it must do so while avoiding the creation of excessive crankcase suction which can be associated with loss of oil pump operation. Further, a device which meets the above objectives must do so without modification to the crankcase.